The present invention relates to a sheet forming apparatus which is applicable to a biaxially stretching film manufacturing system, a sheet manufacturing system, etc.
One example of the sheet forming apparatus of the prior art is illustrated in a cross-section side view in FIG. 1, and temperature distributions on or in the sheet being processed in the apparatus shown in FIG. 1 are illustrated in FIG. 2. In FIG. 1, reference numeral 1 designates an extrusion die which extrudes thermoplastic resin in a sheet shape, and an extruded wheet 3 is cooled by blowing pressurized air from an air knife 2 onto the sheet 3. Reference numeral 4 designates a first cooling roll, which is rotatably supported on a shaft extending from side plates (not shown) and which serves to uniformly cool the sheet 3 by feeding a water coolant to its interior.
Reference numeral 5 designates a second cooling roll having a similar structure to the first cooling roll 4, and the first cooling roll 4 and the second cooling roll 5 jointly nip the sheet 3 at a point a to effect double-side polishing of the sheet 3, and subsequently, the sheet 3 is successively cooled while passing around a third cooling roll 6 and a cooling drum 8 within a water tank 7. It is to be noted that the second cooling roll 5 and the third cooling roll 6 are movable in directions b and in directions c, respectively.
In addition, reference numeral 9 designates a dewatering device, which is constructured of air nozzles for blowing away moisture from a wet sheet 3 coming out of the water tank 7, and the like. Reference numeral 10 designates a drawing roll, which draws up the sheet 3 by nipping it between a nip roll 11 and the drawing roll 10 to feed the sheet 3 to an apparatus in a subsequent step such as, for example, a width expanding machine. Also, the nip roll 11 is movable in directions d.
However, in the apparatus shown in FIG. 1, if the yield, that is, the conveying speed of the sheet 3 extruded from the die 1, is varied or the thickness of the sheet 3 is varied, then because the temperature of the sheet 3 at the point a which is the nip point between the first cooling roll 4 and the second cooling roll 5 will also differ correspondingly, it is necessary to regulate the cooling capacities of the first cooling roll 4 and the second cooling roll 5 in accordance with the difference in the sheet temperature. However, in some cases the regulation of the cooling capacities of the first cooling roll 4 and the second cooling roll 5 cannot be carried out sufficiently to maintain a sheet temperature at which double-side polishing can be achieved.
According to our measurement of the temperature distributions along one principal surface A, the opposite principal surface B and a center C of the thickness of the sheet 3, as shown in FIG. 2 the surface A is at first quickly cooled by the first cooling roll 4 (time interval I), but as the sheet 3 is transferred onto the second cooling roll 5 the surface A abruptly rises in temperature and remelts (time interval II), and sometimes glaze formed on the surface A by the first cooling roll 4 may disappear and creases may be generated on the surface A.
Thereafter, the surface A is successively cooled while being subjected to lowering of its temperature by the third cooling roll 6 (time interval III), raising of the temperature during the period before making contact with the cooling drum 8 (time interval IV) and lower of the temperature within the water tank 7 (time interval V).
Also with regard to the surface B, a remelting phenomenon similar to that occurring on the surface A will appear when the sheet 3 moves from the second cooling roll 5 to the third cooling roll 6, that is, in the time interval III. Therefore, the prior art apparatus has the shortcomings that not only is the time required before the temperatures on the surfaces A and B and at the center C become approximately equal to each other, but also the cost is high because the third cooling roll 6 is provided besides the first cooling roll 4 and the second cooling roll 5.